1. Field of the Invention
The present invention relates to a copying apparatus and, more particularly, to a copying apparatus having a sheet holding device designed so that printed-out sheets, discharged one after another from an image forming device of an electrophotographic copying machine, printer, or the like, are reversed, if necessary, and loaded in groups on a sheet holding plate.
2. Description of the Prior Art
Conventionally, a sheet holding device for holding printed-out sheets discharged from an image forming device includes, as Japanese Published Unexamined Patent Application No. 203054/1984 discloses, a shift mechanism for shifting printed-out sheets in groups of a given number of sheets each in side by side relation so as to enable the sheet holding plate to hold a large number of sheets, and an elevating mechanism for moving the sheet holding plate up and down to keep the height of sheet loading on the holding plate at a constant level. If sheets printed out sequentially from a first page are to be arranged in order of pages, as U.S. Pat. No. 3,833,911, issued Sept. 3, 1974 to Caldwell et al., teaches, it is necessary to provide a reversal mechanism by which each sheet discharged from the image forming device, with the image side up, is reversed so that the image side is turned downward.
However, with any such prior-art sheet holding device, in which the sheet reversal mechanism, shift mechanism, and elevating mechanism are integrally combined together, it is impossible to separate one of the mechanisms for combination with such other device as sorter or the like.
Where a sheet holding device adapted to be attached to a printer or copying system in which copying is made beginning with a first page is employed, there must be provided a face-down path along which each copy sheet is transported, with its image side turned downward, for delivery onto a tray in the case of single-side copying, and a face-up path along which each even-paged copy sheet is transported with its image side up for delivery onto the tray in the case of double-side copying. Whichever one of the paths may be employed for transport of sheets, each sheet is finally delivered through a pair of delivery rollers onto the tray.
In this case, however, the trouble is that depending upon whether each sheet is passed through the face-up path or the face-down path, the angle of the sheet relative to the pair of delivery rollers varies, and accordingly the delivery angle of the sheet through the roller pair varies considerably, with the result that the condition of sheet alignment on the tray varies between the case of the one path being used and the case of the other path being used. Another problem is that if the direction of sheet delivery is inclined excessively downward relative to the tray, sheets being delivered are likely to push outward those already loaded on the tray, while, if the direction is inclined excessively upward relative to the tray, .it is likely that some air is included between the sheets being delivered and those on the tray, so that the dropping on the tray of the sheets being delivered becomes irregular, the condition of sheet alignment being thus unfavorably affected.
In any sheet holding device for holding printed-out sheets discharged one after another from an image forming device, the loading capacity of the tray is determined by the depth or distance between the nip portion of the delivery roller pair and the upper surface of the tray. Therefore, in order to increase the loading capacity, it is necessary to set the tray deeper. On the other hand, however, in order to achieve satisfactory alignment of sheets loaded, it is necessary to set the tray less deep. Where a fixed tray system is employed, therefore, the loading capacity is naturally limited because of the relative position of the nip portion and the tray.
For this reason, as aforesaid Japanese Published Unexamined Patent Application No. 203054/1984 indicates, there has been conventionally employed a system such that the tray is lowered according to the volume of sheets loaded on the tray. With such system, it is a usual practice that the timing for the lowering of the tray is determined by a level detection signal from sensor means for detection of upper surface level of sheets loaded on the tray. However, it is possible that if any sheet is obliquely delivered onto the tray or if a curled sheet is delivered, not all delivered sheets may be held in order on the tray. In such case, the trouble is that the level sensing means will maintain itself in output condition for detection signalling so that the tray may be lowered more than required, sheet alignment being thus disturbed.